1. Field Of The Invention:
This invention relates to urea-zinc oxide fertilizers and methods of preparing them. More particularly, this invention relates to the urea particles coated with a particular zinc oxide composition, which exhibit excellent uniformity and adherence of the zinc oxide coating, even with relatively high amounts thereof.
2. Description Of The Prior Art:
Urea is widely used in many fertilizer applications as a source of nitrogen. In many instances it is the preferred nitrogen source, since it contains a higher percentage of nitrogen as compared with many other available nitrogen-containing materials. Furthermore, urea may be made into a variety of particulate forms, such as granules, crystals, or prills, which are convenient to handle.
Zinc has been used with a variety of fertilizer compositions, since it is one of the micronutrients identified as essential to all living things, and therefore a very desirable trace element. However, in many applications, such as the correction of soil zinc deficiences and the growth of particular crops, such as rice, corn, or the like, zinc is often used in relatively large amounts on the order of 6 to 8%. Heretofore, however, no suitable method has been devised for incorporating such relatively large amounts of zinc into particulate urea fertilizers. In the past, it had been thought necessary to use water-soluble zinc additives, such as zinc sulphate, zinc nitrate, zinc chloride, zinc carbonate, zinc complexes, and chelates such as zinc-EDTA chelate. However, water-soluble zinc salts are highly hygroscopic, and therefore generally unsuitable for use in particulate fertilizer compositions when present in greater than trace amounts.
The percentage of elemental zinc present in various zinc sources varies according to the chemical composition and purity of the zinc source being used. For example, zinc oxide sources usually vary from 70 to 78% zinc, as compared to only 26 to 36% zinc in zinc sulphate, and 5 to 14.5% zinc in chelates of zinc. Thus, for use in fertilizer compositions containing large amounts of zinc, zinc oxide would be preferred due to its high content of elemental zinc.
It is highly desirable to apply zinc in a single step together with other fertilizing compounds, due to the labor savings achieved. Prior art methods for adding zinc salts to fertilizer compositions for such one-step application have included dry blending, coating, and solution dispersion methods. Two major disadvantages detract from all of these methods, viz. hygroscopicity and the difficulity of obtaining a uniform product.
Dry blending of zinc salts with particulate fertilizer compositions is generally unsatisfactory, since combinations of zinc salts and urea are usually hygroscopic and thus absorb moisture from the air and agglomerate into an unusable solid cake. Furthermore, powdery zinc compounds readily separate from the particulate material resulting in a waste of zinc, often interfering with the proper operation of fertilizer dispensing machinery and causing nonuniform application to crops.
While the use of zinc compounds in a liquid dispersion is at present probably the most widely used technique for applying zinc fertilizer compositions, the water-insolubility of zinc oxide poses serious drawbacks. The handling of such dispersions is complicated by their heterogenity, which requires mixing prior to any fluid transfer during handling or application to assure a uniform suspension.
In one attempt to overcome the disadvantages faced by the prior art, several methods of coating zinc compounds on particulate fertilizer compositions have been described. However, the inherent highly hygroscopic properties of the zinc salts still cause problems, since particulate fertilizer compositions coated with these zinc salts tend to absorb moisture from the air and agglomerate into an unusable solid cake. Attempts to slow the rate of water absorption by the use of a clay binder or other exterior coating have not been sufficient to counteract the increased rate of water absorption. Furthermore, it is extremely difficult to obtain uniform coating of zinc salts on urea. The presence of an uneven coating results in exposed areas which provide sites for the urea particles to cake or stick together, and the zinc compounds tend to powder off from the urea particles, forming a zinc powder separate from the particulate material to be applied. Thus, the disadvantages of the dry blending technique are not overcome in a wholly satisfactory manner.
In one particular attempt to overcome the drawbacks of coating fertilizers with zinc and related compounds, Philen, Jr., et al U.S. Pat. No. 3,423,199 describe a process wherein granules of hygroscopic fertilizer salts are coated with micronutrient powders which, when wetted with water and/or steam, react with the fertilizer constituents to form in situ stable complexes. Fertilizer granules, which may include urea, are first dried to a relatively low level of moisture content, and then mixed vigorously for at least 3 minutes with a micronutrient compound ground to a fine particle size, preferably 95% or more passing through a standard 325 mesh screen, which corresponds to a particle size of 44 microns or less. The micronutrient is incorporated intimately into the structure of the granule surface, rather than in a shell surrounding the granule. Several problems remain using this process; for example, the adherence when using zinc oxide is stated to be less satisfactory in continuous tests than in batch tests. Furthermore, complete coverage of the binding agent on the fertilizer could not be obtained in a continuous mixer, although such coverage was attainable in batch tests. When combining an insoluble micronutrient source with macronutrient material, a certain ratio of micronutrient could not be exceeded in order to obtain adequate solubility in the solution resulting from the dissolution of the macronutrient material. In addition, it would be desirable if such coatings could be applied without the necessity of subsequent treatment with water, since these particulate fertilizers must be kept dry to be free-flowing, and since they have hygroscopic properties, which tend to absorb and hold the water applied in any subsequent treatment.
Those concerned with the development of solid, particulate, zinc-containing fertilizer compositions have long desired a particulate urea-zinc fertilizer which could be handled in the same manner as particulate urea fertilizers. Furthermore, it would be highly desirable if such particulate fertilizers could be prepared by a relatively simple process which did not significantly increase the hygroscopicity of the resultant product. The present invention fills such needs.